So in stead of a tiny rotating seal, we have a huge rotating seal. This is not an improvement.

It seems that ORNL also had ideas to use a purge gas stream of cover gas (dry helium) to purge the pump seal with controlled inleakage. But power failure as you say, or just failure of the seal because of its extreme environment (rotating seals fail all the time even in kinder environments), then there's trouble.

There is a similar problem with magnetic bearings: power failure means no bearing, so the pump risks destroying itself, worst case with a big contamination of the plant. Usually catcher bearings are used, but how do we know the catcher bearing will work if it has stayed submerged in corrosive fluxing fuel salt for years? If we know the latter to be the case, we might as well skip the magnetic bearing and use a reliable mechanical bearing (tungsten carbide-silicon carbide, etc.).

We could use seal upong seal upon seal like Lars suggests, but it's not really elegant and rather an inspection nightmare to have many nested seals. A seal that you can't inspect isn't worth as much.

So in stead of a tiny rotating seal, we have a huge rotating seal. This is not an improvement.

It seems that ORNL also had ideas to use a purge gas stream of cover gas (dry helium) to purge the pump seal with controlled inleakage. But power failure as you say, or just failure of the seal because of its extreme environment (rotating seals fail all the time even in kinder environments), then there's trouble.

There is a similar problem with magnetic bearings: power failure means no bearing, so the pump risks destroying itself, worst case with a big contamination of the plant. Usually catcher bearings are used, but how do we know the catcher bearing will work if it has stayed submerged in corrosive fluxing fuel salt for years? If we know the latter to be the case, we might as well skip the magnetic bearing and use a reliable mechanical bearing (tungsten carbide-silicon carbide, etc.).

We could use seal upong seal upon seal like Lars suggests, but it's not really elegant and rather an inspection nightmare to have many nested seals. A seal that you can't inspect isn't worth as much.

The vibration force is a strong (cubic?) function of velocity so we could power the magnetic bearings using the rotor rotation itself which would generate a current proportional to the rotational velocity so the rotor will be better centered as it coasts down than it was during normal operation.

I don't think it is hard to insert a sensor between the seals.

The third seal would be around the whole motor and is intended to limit spread of fission products in the event of failure.

Maybe the vat is sealed, the PHXs are built into the side of the vat and fixed to it, and the fuel salt is pumped by a combination of natural convection and Coriolis forces. I know that it's hard to make the design work for just natural convection alone. But in combination with Coriolis forces it might be possible.

The sealed rotating vat would act as a centrifugal pump impeller for the secondary salt, with an axial intake at the bottom and peripheral exits near the top.

My original idea with spinning the fuel salt container was to get rid of the need for any rotating seals, by using magnetic bearings up above the fertile salt. There would be no seal between fissile and fertile. But the change in buoyancy when the reactor stopped, and all the salt from the ( non rotating ) heat exchangers ran down into the vat, would bring a whole lot more problems with aligning the gearing that spun the vat.

Maybe the vat is sealed, the PHXs are built into the side of the vat and fixed to it, and the fuel salt is pumped by a combination of natural convection and Coriolis forces. I know that it's hard to make the design work for just natural convection alone. But in combination with Coriolis forces it might be possible.

The sealed rotating vat would act as a centrifugal pump impeller for the secondary salt, with an axial intake at the bottom and peripheral exits near the top.

ORNL's MSBR had primary pump power on the order of several thousand kWe. The HX was a high pressure drop type because it must be very compact.

At some point the rotation has to stop; the secondary heat exchangers and steam generators don't rotate. So you're into the tricky design territory of large rotating seals, in an extreme environment.

So in stead of a tiny rotating seal, we have a huge rotating seal. This is not an improvement.

It seems that ORNL also had ideas to use a purge gas stream of cover gas (dry helium) to purge the pump seal with controlled inleakage. But power failure as you say, or just failure of the seal because of its extreme environment (rotating seals fail all the time even in kinder environments), then there's trouble.

There is a similar problem with magnetic bearings: power failure means no bearing, so the pump risks destroying itself, worst case with a big contamination of the plant. Usually catcher bearings are used, but how do we know the catcher bearing will work if it has stayed submerged in corrosive fluxing fuel salt for years? If we know the latter to be the case, we might as well skip the magnetic bearing and use a reliable mechanical bearing (tungsten carbide-silicon carbide, etc.).

We could use seal upong seal upon seal like Lars suggests, but it's not really elegant and rather an inspection nightmare to have many nested seals. A seal that you can't inspect isn't worth as much.

The vibration force is a strong (cubic?) function of velocity so we could power the magnetic bearings using the rotor rotation itself which would generate a current proportional to the rotational velocity so the rotor will be better centered as it coasts down than it was during normal operation.

I don't think it is hard to insert a sensor between the seals.

The third seal would be around the whole motor and is intended to limit spread of fission products in the event of failure.

All of the large magnetic bearings I've seen use high frequency positional techniques to keep the bearing exactly aligned at all times. Apparently it's rather tricky and requires high frequency software and electromagnet response.

Canned rotor pumps typically put the bearings close to the rotor, so for an MSR everything, rotor, bearings, stator, will be inside the hot cell. This seems like a difficult environment for software and electronics. Then again the AP1000 design also has canned rotor pumps and they deal with a lot of gamma radiation there.

So in stead of a tiny rotating seal, we have a huge rotating seal. This is not an improvement.

It seems that ORNL also had ideas to use a purge gas stream of cover gas (dry helium) to purge the pump seal with controlled inleakage. But power failure as you say, or just failure of the seal because of its extreme environment (rotating seals fail all the time even in kinder environments), then there's trouble.

There is a similar problem with magnetic bearings: power failure means no bearing, so the pump risks destroying itself, worst case with a big contamination of the plant. Usually catcher bearings are used, but how do we know the catcher bearing will work if it has stayed submerged in corrosive fluxing fuel salt for years? If we know the latter to be the case, we might as well skip the magnetic bearing and use a reliable mechanical bearing (tungsten carbide-silicon carbide, etc.).

We could use seal upong seal upon seal like Lars suggests, but it's not really elegant and rather an inspection nightmare to have many nested seals. A seal that you can't inspect isn't worth as much.

The vibration force is a strong (cubic?) function of velocity so we could power the magnetic bearings using the rotor rotation itself which would generate a current proportional to the rotational velocity so the rotor will be better centered as it coasts down than it was during normal operation.

I don't think it is hard to insert a sensor between the seals.

The third seal would be around the whole motor and is intended to limit spread of fission products in the event of failure.

All of the large magnetic bearings I've seen use high frequency positional techniques to keep the bearing exactly aligned at all times. Apparently it's rather tricky and requires high frequency software and electromagnet response.

Canned rotor pumps typically put the bearings close to the rotor, so for an MSR everything, rotor, bearings, stator, will be inside the hot cell. This seems like a difficult environment for software and electronics. Then again the AP1000 design also has canned rotor pumps and they deal with a lot of gamma radiation there.

Propagation time to send the sensor data 10 meters is awfully small compared to mechanical time so there is no rationale for the s/w to be in this tough environment. We do need to be able to send a signal (hence insulate a wire and for that matter have a wire - I don't suppose aluminum wire will work . The sensors do need to deal with the environment. We have to have sensors for safety so the question is whether the wider bandwidth requirements of active controls make this any more difficult.

No electronics in the hot cell. I've been told that electronics are even less resistant to radiation than people.

But this is not a major problem. It means that there will be large bundles of conductors and fiber optics coming out of the hot cell, just like telephone COs have. There will be wiring failures in the hot cell, and the seal on these wires as they leave the hot cell will be difficult to mess with, so we need to have lots of nicely labelled spare circuits to use as the originals fail, and standardized connectors which we can decouple and recouple remotely... just like telephone COs have. Conductors passing high powered AC waveforms, like the phases of a BLDC motor, have to be twisted and perhaps shielded. This is mostly doable.

I would like to hear about people's experiences with hermetically sealing a bundle of wires, especially wires that must be shielded like coax. I have personally had an extended and extremely unpleasant battle with this problem, and I would love to hear that someone has experience with a plug that can pass hundreds or thousands of conductors and pass a helium leak test. My guess is that the trick is bare solid wires passing through a glass plug, where the CTE of the wire is matched to the glass. Something like pure tungsten (4.3 ppm/K) and Pyrex/Borofloat 33 (3.25 ppm/K). For a special application like this, I'm sure Schott would be happy to make some glass with a bit less SiO2 and a correspondingly higher and better matched CTE.

I would think fiber optics would be "yellowed" by the extremely high radiation doses in the vault, signals would degrade quickly. As implied above, wiring insulation materials would have to be screened carefully.

No electronics in the hot cell. I've been told that electronics are even less resistant to radiation than people.

People are actually fairly resistant to radiation; not that you would get folk like Caldicott to recognize that fact.

They are resistant against the level of radiation found in every natural environment on the planet. But inside a hot cell of a molten salt reactor, we're talking about radiation levels in excess of 1000 Sv/hour, some parts over 10000 Sv/hour, depending on the design and proximity to fuel salt. This is deadly for any human on an exposure scale of mere seconds to minutes.

If a canned rotor pump is used, some sensors are needed in a high radiation field, especially if magnetic bearings are used. Wiring is probably not a problem, with many conductive metals and nonmetals being resistant to high radiation levels, and ceramic electrical insulation coatings are available. Some glasses may also be available with high radiation resistance, and good optical properties, for fiber optics. In fact, fiber optics with video outside the hot cell look a lot more robust than actively cooled video cameras and such inside the hot cell.

My understanding is that glasses loaded with Cerium Oxide are radiation resistant.

I have, right in front of me at the moment, a couple of lens elements with a high fraction of thorium oxide. These turn slightly brown over time, as the alpha particles damage the glass. To preserve the brown, it's important not to expose the elements to direct sunlight, which apparently restores them in a couple of hours.

I concur that a fiber bundle going to an external video camera sounds more robust than a bullet cam.

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